When an oil well is initially completed the downhole pressure may be sufficient to force the well fluid up the well tubing string to the surface. The downhole pressure in some wells decreases over time, and some form of artificial lift is required to get the well fluid to the surface. One form of artificial lift involves suspending a centrifugal electric submersible pump (ESP) downhole in the tubing string. The ESP provides the extra lift necessary for the well fluid to reach the surface. An ESP has a large number of stages, each stage having an impeller and a diffuser. In gassy wells, or wells which produce gas along with oil, there is a tendency for the gas to enter the pump along with the well fluid. Gas in the pump decreases the volume of oil transported to the surface, decreases the overall efficiency of the pump, and reduces oil production.
A progressing cavity pump is another type of well pump which typically comprises a helical metal rotor rotating inside a correspondingly formed helical elastomeric stator. The liquid being pumped lubricates the contact surface between the helical rotor and the stationary stator. Gas entering the pump not only reduces its pumping efficiency, but also prevents the liquid from continuously lubricating the rotor and stator surfaces while being forced through the pump. The stator deteriorates quicker when not lubricated, thereby increasing pump maintenance and repair frequency.
One example of a prior art progressing pump assembly 10 is shown in a side partial cross sectional view in FIG. 1. Pump assembly 10 is suspended from tubing 12 in a well in order to pump well fluid to the surface through the tubing 12. Pump assembly 10 includes a progressing cavity pump 14 having a helically shaped rotor 16 rotating within an elastomeric stator 18. An inlet 20 is located at the lower portion of progressing pump 14 where liquids enter pump 14. An outlet 24 is located at the upper portion of progressing cavity pump 14 for discharging the liquids up the string of tubing. Liquids entering pump 14 flow into a double helical cavity 22 between rotor 16 and stator 18. Rotor 16 rotates so that the helical shape of rotor 16 and stator 18 force liquid to travel up pump 14. The liquid in cavity 22 is forcibly moved as portions of cavity 22 rise along rotor 16 to outlet 24, where the liquid is discharged above pump 14 into the string of tubing 12 leading to the surface. The liquid leaves a thin layer of liquid on the surfaces of rotor 16 and stator 18 as the liquid in cavity 22 travels up rotor 16 through pump 14. The thin layer of liquid left on the surfaces of rotor 16 and stator 18 acts as a lubricant, increasing the operational lifespan of rotor 16 and stator 18.
A motor (not shown) drives the rotor 16 from below pump 14 via a flex shaft 28; the flex shaft 28 is shown attached to the lower end of the rotor 16. The upper end of the flex shaft 28 orbits with the lower end of the rotor 16 while the flex shaft 28 lower end rotates concentrically with the motor shaft. As seen in FIG. 1, clearance is provided in the coupling 26 between the flex shaft 28 and rotor 16 to accommodate vertical force fluctuations experienced by the rotor 16 during pumping. A housing 30 encloses the flex shaft and provides a conduit for wellbore fluids flowing to the pump inlet 20.